Power inverters can greatly improve the overall efficiency, performance and quality of signals of a power system. Conventionally, two-level inverters are widely used for their low cost and minimal complexity. However, high voltage stress, high harmonic distortions, and instantaneous voltage change rate are some of the challenges facing a two level inverters.
Multilevel (ML) inverters can potentially address these challenges. ML inverters can be used for high-power, high voltage applications due to inherent advantages, such as reduced voltage stress, lower harmonics, lower instantaneous voltage change rate dv/dt and lower common-mode voltage. Modular multilevel converters are used for renewable energy applications. ML inverters are extensively used in high-power applications with medium voltage levels. The applications include use in laminators, mills, conveyors, pumps, fans, blowers, compressors, and so on industry is another driver for advancing multilevel inverter.
ML inverters can generate output voltages with extremely low distortion and lower change rate. Also, ML inverters draw input current with very low distortion, and generate smaller common-mode (CM) voltage, thus reducing the stress in motor bearings. In addition, using sophisticated modulation methods, CM voltages can be eliminated.
ML inverters generally use an array of power semiconductors and capacitor voltage sources to produce output voltages with stepped waveforms. The commutation of the switches permits the addition of the capacitor voltages, which reach high voltage at the output, while the power semiconductors can only withstand reduced voltages. The switches are typically controlled by a modulator implemented in digital circuit.
ML inverter circuits have been around for more than three decades. One cascade inverter connects separately DC-sourced full-bridge cells in series to synthesize a staircase ac output voltage. A diode-clamped inverter uses diodes to block the sources. The diode-clamped multilevel inverter is also called a neutral-point clamped (NPC) inverter as shown in FIG. 1 with an input voltage VCC, and output levels a, b, and c for a conventional inverter.
There are a number of topologies for the analog circuits. In diode-clamped capacitor array, the DC-bus voltage is split into N levels by N−1 series connected bulk capacitors, and diodes are used to clamp the voltage at the switch to output. The middle point n of the capacitor array can be defined as the neutral point.
Space vector modulation (SVM) is a multilevel modulation technique that is widely used. It has been suggested that that the correct designation for the analytical tool to analyzing electrical machines has to be space phasor, instead of a space vector. The space phasor concept is mainly used for current and flux in analysis of electrical machines.
Recent progress in the semiconductor technology has made available fast commutating electronics power devices to be used with high voltages and currents, so extending the application field of voltage source inverters (VSI). Actually, it is not still possible to employ conventional VSI in the high power range due to commutation difficulties and reverse recovery high voltages.
Multilevel power converters are an interesting emerging technology for medium and high power applications including the fields of renewable energy sources and power quality issues as active filtering and reactive power compensators, e.g., static volt ampere reactive (VAR) compensators.
One fast modulation procedure for multilevel SVM three phase converters projects the line phases to a 3-dimensional Euclidean space, and uses a linear transformation to convert the switching states to a 2-dimensional non-orthogonal coordinate. The procedure is computational efficient. However, it requires that a 3 phase reference signal is generated.
Another method uses a modulator design with an iterative search procedure and can take up to N steps for an N−1 level modulator to reach the solution. The computation complexity is significant because of the non-linear trigonometrical operations. It also suffers high latency when the number of levels becomes high.